We have found a high frequency (1-2 percent) of non-overlapping antigen-binding cells for each of two antigens, beta-galactosidase and horseradish peroxidase. About 1 percent of beta-galactosidase (BGZ)-binding cells also can bind horseradish peroxidase (HRP), suggesting a random assortment of specificities on multipotential cells. Our major objectives are discovering how these multispecific precursor cells arise and differentiate to unispecific producing cells. We will purify BGZ-binding cells by binding to a pseudo-substrate affinity column and eluting with excess substance, free BGZ, or a variety of other tactics. Purified cells from unimmunized animals will be compared to those from primed animals to determine whether specificity restriction has occurred. By affinity fractionation of binding T and B cells, followed by functional assay, we will try to examine the relevance of antigen-binding to subsequent immune responsiveness. Several questions about the nature of receptors will be asked: their origin in ontogeny and the frequency of double binding fetal cells; receptor biosynthesis - are they all synthesized by the cell bearing them?; how many receptors are there on T and B cells?; how does antigen affect their surface distribution? The T-function of thymus antigen-binding cells will be studied upon adoptive transfer and in helper experiments. The genetic and clonal basis of antigen-binding in T and B cell lines will also be investigated. BIBLIOGRAPHIC REFERENCES: Miller, A. A critical examination of the numerology of antigen-binding cells. Evidence for multiple receptor specificities on single cells. Ann. Immmunol. 127c, 1977, in press. Krolick, K.A., Wisnieski, B.J. and Sercarz, E.E. Differential lateral mobility of IgM and IgG receptors in mouse B lymphocyte membranes. Fed. Proc. 36 (1977), in press.